Micromechanical optical switch

ABSTRACT

The present invention relates to optical MEMS components, and in particular, to a micromechanical optical switch. A removable layer is used during fabrication to define the gap between optical waveguides and a moveable element in the form of a mirror that is moved between states. This provides a high speed, low-power optical switch that is readily manufacturable.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/645,203filed Aug. 25, 2000 U.S. Pat. No 6,411,754. The entire teachings of theabove-referenced application are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Optical switches have been developed using guided wave devices orfree-space mechanical devices. Guided wave devices use waveguides,whereas free-space devices use optical beams in free space with movableoptical elements such as mirrors or lenses.

Guided wave devices typically divert light from one arm of the deviceinto the other by changing the refractive index of one of the arms ofthe device. This is typically done using electrical, thermal, or someother actuating mechanism.

The free-space approach has an advantage over the guided-wave approachin some applications. It has very low cross talk because the waveguidesare physically isolated from one another and coupling cannot occur. Theprincipal source of cross talk in this approach is scattering off themovable optical element. In addition, free-space devices arewavelength-independent and often temperature-independent.

Existing designs employ mirrors positioned at the intersection of inputfibers and output fibers. Due to the spreading of the light beam as itleaves the waveguide and travels toward the mirror large mirrors areused that require mounting and angular placement accuracy. There can besignificant difficulty in actuating such a relatively large structurequickly and accurately at the switching speeds required for opticalcommunication systems.

Thus, a need exists for an optical switch having the advantages of thefree-space approach, without the disadvantage of existing designs.

SUMMARY OF THE INVENTION

The present invention relates generally to the field of optical MEMS(micro-electro-mechanical system) and more specifically to the use offabrication techniques used in making micromechanical devices tofabricate high speed optical MEMS for optical communication networks.The method employs the use of a removable layer that is formed betweenan optical waveguide and a movable switch element that has been formedover the removable layer and the waveguide. The removable or sacrificiallayer is preferably formed using a conformal material such as parylene(poly-para-xylene).

A preferred embodiment of the invention can use a silica substrate withoptical waveguides formed therein as the initial structure in themanufacture of the optical MEMS. After formation of a trench in thesubstrate to define a gap between waveguide elements, a first mask isused to define the routing wire geometry on the upper surface of thesubstrate. The trench can have a width of about 3 to 20 μm. Subsequentlythe removable layer is formed followed by the use of a second mask forfabrication of a switch element layer.

After patterning of the switch element layer, a spacer layer, preferablya photoresist layer, is spun on the surface and patterned using a thirdmask. A metallization layer, preferably an evaporated layer of copper,is deposited and a further photoresist layer is formed using a two maskexposure sequence. This defines a mold for fabrication of a platinglayer. In a preferred embodiment of the invention, nickel iselectroplated into the mold to form an integral electrode structure.

The removable layer is then preferentially etched to release the switchelement which has been fabricated with a spring that supports the switchrelative to the substrate. The characteristics of the spring define thespeed and pull-up voltage of the switch element.

The electrodes are used with an overlying actuating electrode structureto actuate movement of the switch element between states. A preferredembodiment of the invention uses a reflective element or mirror that ismoved from a first position, in which light from a first waveguide isreflected by the mirror into a second waveguide, to a second position inwhich the mirror is translated vertically to permit light to passthrough the gap on a linear optical path into a third optical fiber thatis aligned along a single axis with the first fiber. The waveguidesand/or the trench can be filled with air or in another embodiment can befilled with a fluid. Further details regarding the use index matchingfluids are described in International Application No. PCT/US99/24591filed on Oct. 20, 1999, the entire contents of which is incorporatedherein by reference.

Another preferred embodiment of the invention involves the fabricationof an array of switches on a single substrate that can serve as amonolithic array, or alternatively, the substrate can be diced toprovide separate switches or arrays of switches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1K illustrate a method of fabricating an optical switch inaccordance with the invention.

FIG. 2 is a schematic cross-sectional view of a preferred embodiment ofan optical switch in accordance with the invention.

FIG. 3 illustrates the switching time of a preferred embodiment of theinvention.

FIG. 4 is a top view of an optical switch in accordance with theinvention.

FIG. 5 is a top view of another preferred embodiment of the invention.

FIG. 6 is a top view of another preferred embodiment of the invention.

FIG. 7 is a top view of another preferred embodiment of the invention.

FIG. 8 is an array of optical switches made in accordance with theinvention.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred method of fabricating a micromechanical optical switch isillustrated in the process sequence of Figures 1A-1K. This method beginswith a substrate 10, such as silica wafer, in which a cavity or trench12 has been formed by standard etching techniques. As described indetail below, the substrate can include one or more optical waveguides.A first mask pattern is formed for conductive routing lines bydepositing a photoresist layer 14 and selectively removing portionsthereof to define the metallization pattern 16,18.

In Figure 1B, a conductor layer 20 is formed, preferably by evaporationof a metal. In a preferred embodiment, a plurality of layers including atitanium layer, a nickel layer and a gold layer are used to provide thedesired electrical and mechanical properties. In an example, thetitanium layer has a thickness of 500 Å, the nickel layer has athickness of 1500 Å and the gold layer has a thickness of 3000 Å. Arinse can be used to remove excess metal and the photoresist andoverlying metal is than removed to provide conductive routing lines 30shown in FIG. 1C.

A sacrificial layer 40 is then formed on the device, preferably a layerof parylene having a thickness in the range of 0.5 μm to 25 μm,depending on the width of the trench 12. This is followed by a layer 42,of a reflective material such as gold. In this particular example theparylene layer has a thickness of 3.5 μm and the gold layer is depositedin 0.21 μm steps for a total thickness of about 2.0 μm.

In FIG. 1E, the reflective layer 42 is patterned to form a reflector ormirror 50. A photoresist (AZ9260) is spun, baked, exposed, and developedto define the mask pattern 55 and the exposed gold is removed with aTransene TFA etchant. Next, after removal of the mask, a directional(RIE) etch in an O₂ plasma is used to remove the parylene with themirror 50 acting as a mask and leaving a residual layer 62 as seen inFIG. 1F.

Another photoresist layer 70 is then deposited and patterned to defineanchor positions 72 (FIG. 1G). Layer 70 has a specified thickness todefine a gap between the suspended structure that will support themirror relative to the substrate. The gap is preferably in a rangebetween 5 and 20 μm and in this particular example is about 15 μm. Thesize of the anchor openings can be measured to verify proper alignmentand preferably each opening has an area in a range between 40 and 50μm².

FIG. 1H illustrates formation of a metal seed layer having a thicknessin a range of 1000 to 50,000 Å. In this particular example, a copperlayer having a thickness of 5000 Å is deposited by evaporation.

Another photoresist pattern 90 is formed as shown in FIG. 1I using adigitally controlled oven at 45° for 4 hours. A two step exposuresequence is used to minimize variations in thickness of the photoresist.Features 92 of the photoresist layer 90 are used to define electrodes inthe suspended membrane that are used in actuating movement of the switchelement. As shown in FIG. 1J, a nickel layer 100 is formed, preferablyusing an electroplating process in which three separate regions, thefirst region 102 being at the anchor, the second region at electrodes104, 106 and the third region at the mirror 108. In a preferredembodiment of the invention a nickel sulfate solution is used with acurrent of 17.5 mA at 45° C. with a plating time of 30 minutes toprovide a 6 μm thick layer.

As shown in FIG. 1K, the photoresist 90, 92 is removed, the exposedcopper is then etched using ammonium hydroxide and copper (II) sulphateto access the spacer material 70 which is then removed. Finally, adiclorobenzene etch is performed at 150° C. that removes the remainingparylene 61 to release the mirror structure 120.

The above procedure can also be used in fabricating a mirror that can bedisplaced laterally in the trench using a different method of actuationsuch as electrostatic comb drive or thermal actuation which can be usedto provide a bistable switch, for example.

Illustrated in the schematic cross-sectional view of FIG. 2 is anoptical switch 200 in accordance with the invention. An overlyingactuating electrode panel 202 having an actuating electrode 204 that isseparated from the suspended membrane 208 by a gap 218 that ispreferably about 50 μm. Spacers 206 can be made using an oxide toprevent shorting between electrodes 204 and 226 or the mirror surface210.

Note that optional pull down electrodes 216 can also be positioned inthe gap 220 between the fiber cladding 224 and the membrane 208 that ispreferably about 15 μm. The substrate has a thickness 222 that includesthe cladding 224 surrounding the fiber core 212. The fiber core ispreferably about 6 μm. The mirror includes the switching element 214that moves vertically within the trench 228. The pitch of the membranestructure is in a range of 100 to 2000 μm, and is preferably about 500μm. The upper panel 202 can be electrically connected to the lowersubstrate system using flip chip bonding or eutectic bonding. The drivercircuit for the switch can be mounted on substrate or packagedseparately.

FIG. 3 graphically illustrates the vertical mirror displacement as afunction of time for three different pull-up voltages as a function oftime.

FIGS. 4-7 illustrate preferred embodiments of the spring system thatsupports the membrane relative to the substrate. The spring isconfigured to provide a vertical displacement of between 20 and 30 μm.Generally, a higher spring constant in the range of 1.0 to 4.0 N/m alongwith a higher operating voltage in the range of 50-150V results in afaster response time. It is also desirable to minimize or eliminaterotation of the membrane during displacement. FIG. 4 illustrates aspring system 300 having four beams 302 extending from anchors 304 to amembrane connection 306. This particular embodiment has a stiffness of1.99 N/m, a rotation of 0.0025 degrees and a spacing of 10 μm. Thespring system 320 of FIG. 5 has four spring elements 322 extending fromanchors 324 to membrane connectors 326. This embodiment has a higherstiffness at 4.1 N/m, a smaller rotation at 0.0015 degrees and a 10 μmspacing. The embodiment 340 of FIG. 6 has four spring elements 342connected at anchors 344 and connected at 346. The system has astiffness of 2.8 N/m, a rotation of 0.002 degrees and a smaller spacingat 5 μm. The system 360 of FIG. 7 has stiffness of 4.49 N/m, no rotationand a 10 μm spacing.

FIG. 8 illustrates an array 400 of switches fabricated in accordancewith the invention. The array can be 8×8, 32×32, 64×64 or any otherdesired configuration as needed for a particular application. In thisparticular embodiment an 8×8 having input fibers 402, a diagonallypositioned array of switch elements that either reflect light from theinput fibers to output fibers 406, or allow light to pass directlythrough the trench to output fibers 408. The output fibers 406 can beorthogonally arranged relative to fibers 402, or they can be arranged atsome other oblique angle.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. An optical switch comprising: a substrate havinga first optical waveguide having an output surface and a second opticalwaveguide having an input surface, the input surface and the outputsurface being separated by a cavity; an optical switch elementpositioned within the cavity and moveable between a first position and asecond position; and a spring mounted actuator that moves the opticalswitch element between the first position and the second position tocontrol transmission of light between the first optical waveguide andthe second optical waveguide.
 2. The optical switch of claim 1 furthercomprising a third optical waveguide coupled to the cavity such that theswitch element couples light from the first waveguide in the firstposition.
 3. The optical switch of claim 1 further comprising an arrayof input waveguides and an array of output waveguides, each pair ofinput and output waveguides having an optical switch element.
 4. Theoptical switch of claim 1 wherein the substrate comprises a silicawafer.
 5. The optical switch of claim 1 wherein the optical switchelement comprises a reflector.
 6. The optical switch of claim 1 whereinthe spring comprises a plurality of spring elements attached at a firstend to the substrate and at a second end to a moveable beam.
 7. Theoptical switch of claim 1 wherein the spring has a stiffness between 1.0and 5.0 N/m.
 8. The optical switch of claim 1 wherein the switch elementmoves from the first position to the second position in less than 1 ms.9. The optical switch element of claim 1 further comprising a controlcircuit that actuates movement of the switch, the circuit operating at avoltage in a range between 50 to 300V.